Calculate the square footage of a square or rectangular room. Measure the length and width of the room in inches. Multiply the figures to get the area, or square footage, in inches. Divide that figure by 144 to get the area in square feet.
Calculate the square footage of a circular room. Measure the diameter and divide that figure by 2 to get the radius in inches. Multiply the radius by itself, so that it is squared. Multiply the squared radius by 3.14. Divide that result by 144 to get the area in square feet.
Calculate the square footage of a triangular room. Measure the base and height (the distance from the apex to the base) in inches. Divide the base measurement by 2. Multiply half the base by the height to get the area in inches. Divide that figure by 144 to get the area in square feet.
Divide the room into squares, rectangles and triangles if you have a room with a more complex shape. Then find the areas of the various parts. Add those figures together to get the total.